Referring to FIG. 11, there is illustrated a first example of prior art permanent magnet magnetic circuits. The circuit includes a U-shaped yoke 2 having opposed legs. A pair of permanent magnets 1A and 1B are attached to the inside surfaces of the yoke legs such that the N pole of one magnet 1A is opposed to the S pole of the other magnet 1B. This arrangement creates a unidirectional magnetic field containing the least number of unnecessary perpendicular magnetic field components.
In the first prior art circuit of FIG. 11, however, a region E1 where the unidirectional magnetic field is produced is surrounded at three sides by the magnetic circuit. This magnetic circuit is not useful where restrictions are imposed on its attachment or location. For example, this first prior art circuit cannot be applied under the requirement that the magnetic circuit should not protrude beyond the lower boundary line F1 of the unidirectional magnetic field generating region E1. Such restrictions arise, for example, when it is desired for a magnetic circuit located outside a vacuum vessel to provide a region E1 of unidirectional magnetic field within the vacuum container interior.
FIGS. 12 and 13 show a second example of the prior art, illustrating a most fundamental magnetic circuit. A plate-shaped permanent magnet 5 has N and S poles disposed at opposite edges. A region E2 of approximate unidirectional magnetic field is available above (or below) one major surface of the plate shaped permanent magnet 5 as seen from FIG. 13. The magnetic circuit can be located only below (or above) the boundary line F2 of the approximate unidirectional magnetic field region E2.
FIG. 14 shows the distribution of lines of magnetic force generated in the second prior art circuit of FIGS. 12 and 13. In this example, the plate shaped permanent magnet 5 is a ferrite permanent magnet having a lateral dimension Lx of 250 nm, a transverse dimension Ly of 300 mm and a thickness Lz of 24 mm.
Assume that X is a horizontal distance from the center of the plate-shaped permanent magnet 5 and the vertical distance H from the major surface of the plate-shaped permanent magnet 5 is fixed to 40 mm. In FIG. 6, broken line curves show horizontal and vertical (or perpendicular) components Bx and Bz of the magnetic flux density as a function of distance X in the range of from 0 to W/2=80 mm. It is understood from the broken line curves in FIG. 6 and FIG. 14 that the second prior art circuit of FIGS. 12 and 13 produces a magnetic field which is far from an ideal unidirectional magnetic field free of perpendicular components since the unnecessary vertical component Bz drastically increases with the increasing distance X from the center of the plate-shaped permanent magnet 5.